Mass vs Molar Mass
We know about mass though we commonly use the concept of weight instead of mass in our daily life. Weight of a person is a product of the mass of his body and gravity of earth. Since this gravitational effect is same all over the earth, it is more convenient to talk in terms of weight. But when we are talking in terms of smaller objects such as molecules and atoms of substance, the concepts of molecular mass and molar mass are used. There are differences in mass and molar mass of a substance that will be clear after reading this article.
In theory, the mass of a substance is its relative mass as it is expressed in terms of the mass of a carbon atom that has been taken as 12. In practice however, the molecular mass of a substance is taken to be the sum of atomic masses of the constituent atoms of the molecule. For example, if we have to calculate the molecular mass of carbon monoxide we have to add up the atomic masses of carbon and hydrogen. As atomic mass of carbon is 12 and that of oxygen is 16, the molecular mass of carbon monoxide is 12+16=28. Similarly, if the substance is carbon dioxide, we know that there are two atoms of oxygen involved, so the molecular mass of CO2 will be calculated as follows: 12+ 16×2= 44
Molecular mass is a dimensionless number.
The mass in grams of one mole of a substance is called its molar mass. Different substances have different molar masses, and this is because atoms of different elements have different number of electrons, protons and neutrons. One mole of a substance contains 6.022 x 1023 atoms of that substance. This is also known as Avogadro’s number. The molar mass of carbon is 12.011g while that of magnesium is 24.3050g.
But when it comes to molar mass of compounds, the first step is to calculate its molecular mass, which is the sum of atomic masses of the constituent elements. Let us take up the example of water.
Molecular mass of water = 2 x 1.00794 (atomic mass of hydrogen) + 15.9994 (atomic mass of oxygen) = 18.0153
Molar mass of a molecular compound = molecular mass in grams/ 1 mole compound
So the molar mass of water = 18.0153/ 1 mole of H2O
Difference Between Mass and Molar Mass
• Mass, or molecular mass of a substance is the mass of one molecule of the substance and is the sum of the atomic masses of the constituent elements
• Molar mass is the mass in grams of one mole of a substance which is the number of atoms in one mole of the substance. It is a constant number called Avogadro’s number.
• While molecular mass is a dimensionless number being expressed relative to the mass of carbon atom, molar mass is expressed in grams
• Many think of both terms being same but in practice, the average mass is not the same as the mass of a single molecule
• When considering small molecules, the difference is miniscule and the two are roughly equal